Muscle Histology

Muscle Histology

Functions of muscle tissue • Movement • Joint stabilization • Heat generation

Special functional characteristics of muscle • Contractility • ability to shorten and thicken (when muscles work they contract that is they get shorter and thicker) • Excitability • ability to respond to stimulus

Special functional characteristics of muscle • Extensibility • ability to stretch (when muscle is relaxed it becomes longer and thinner) *some muscles work while others relax * • Elasticity • ability to return to original length

Types of Muscle Tissue • Skeletal muscle • Cardiac muscle • Smooth muscle

Types of Muscle Tissue • Skeletal • Attach to and move skeleton • 40% of body weight • Fibers = multinucleate cells (embryonic cells fuse) • Cells with obvious striations • Contractions are voluntary • Cardiac:only in the wall of the heart • Cells are striated • Contractions are involuntary (not voluntary) • Smooth:walls of hollow organs • Lack striations • Contractions are involuntary (not voluntary)

Similarities… • Their cells are called fibers because they are elongated • Contraction depends on myofilaments • Actin • Myosin • Plasma membrane is called sarcolemma • Sarcos = flesh • Lemma = sheath

Skeletal muscle Epimysium: surrounds whole muscle Endomysium is around each muscle fiber Perimysium is around fascicle

Skeletal Muscle • Attachments • Tendon ~ strong, tough connective tissue cord, connect muscle to bone *Achilles Tendon (attaches to calf muscle and heel bone) *lumbodorsal fascia (surrounds the deep muscles of the back and trunk) • Origin: the less movable attachment • Insertion: the end of the muscle that attaches to a bone that moves -where the muscle ends • Usually one bone moves while the other remains fixed • Note: origin and insertion may switch depending on body position and movement produced

Attachments continued • Many muscles span two or more joints • Called biarticular or multijoint muscles • Cause movements at two joints • Direct or “fleshy” attachments • Attachments so short that muscle appears to attach directly to bone • Indirect: connective tissue extends well beyond the muscle (more common) • Tendon: cordlike (most muscles have tendons) • Aponeurosis: flat sheet • Raised bone markings where tendons meet bones • Tubercles, trochanters, crests, etc.

Skeletal muscle • Fibers(each is one cell)have striations • Myofibrils are organelles of the cell: these are made up of filaments • Sarcomere • Basic unit of contraction • Myofibrils are long rows of repeating sarcomeres • Boundaries: Z discs (or lines) This big cylinder is a fiber: 1 cell -an organelle

Myofibrils • Made of three types of filaments (or myofilaments): • Thick (myosin) • Thin (actin) • Elastic (titin) ______actin _____________myosin titin_____

Sliding Filament Model __relaxed sarcomere__ _partly contracted_ fully contracted Sarcomere shortens because actin pulled towards its middle by myosin cross bridges “A” band constant because it is caused by myosin, which doesn’t change length Titin resists overstretching

Another pic

EM (electron microscope): parts of 2 myofibrils Labeled and unlabeled

Sarcoplasmic reticulum • Tubules surround myofibrils • Cross-channels called “terminal cisternae” • Store Ca++ and release when muscle stimulated to contract • To thin filaments triggering sliding filament mechanism of contraction • T tubules are continuous with sarcolemma, therefore whole muscle (deep parts as well) contracts simultaneously

Some sites showing animations of muscle contraction • http://entochem.tamu.edu/MuscleStrucContractswf/index.html • http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/muscles/muscles.html

Types of skeletal muscle fibers • Fast, slow and intermediate • Whether or not they predominantly use oxygen to produce ATP (the energy molecule used in muscle contraction) • Oxidative – aerobic (use oxygen) • Glycolytic – make ATP by glycolysis (break down of sugars without oxygen=anaerobic) • Fast fibers: “white fibers” – large, predominantly anaerobic, fatigue rapidly (rely on glycogen reserves); most of the skeletal muscle fibers are fast • Slow fibers: “red fibers” – half the diameter, 3X slower, but can continue contracting; aerobic, more mitochondria, myoglobin • Intermediate: in between

A skeletal muscle contracts when its motor units are stimulated • Amount of tension depends on • the frequency of stimulation • the number of motor units involved

Muscle hypertrophy • Weight training (repeated intense workouts): increases diameter and strength of “fast” muscle fibers by increasing production of • Mitochondria • Actin and myosin protein • Myofilaments containing these contractile proteins • The myofibril organelles these myofilaments form • Fibers enlarge (hypertrophy) as number and size of myofibrils increase [Muscle fibers (=muscle cells) don’t increase in number but increase in diameter producing large muscles] • Muscle atrophy: loss of tone and mass from lack of stimulation • Muscle becomes smaller and weaker Note on terminology: in general, increased size is hypertrophy; increased number of cells is hyperplasia

Intercalated disc__________ Cardiac muscle • Bundles form thick myocardium • Cardiac muscle cells are single cells (not called fibers) • Cells branch • Cells join at intercalated discs • 1-2 nuclei in center • Here “fiber” = long row of joined cardiac muscle cells • Inherent rhythmicity: each cell! (muscle cells beat separately without any stimulation)

Smooth muscle • Muscles are spindle-shaped cells • One central nucleus • Grouped into sheets: often running perpendicular to each other • Peristalsis • No striations (no sarcomeres) • Contractions are slow, sustained and resistant to fatigue • Does not always require a nervous signal: can be stimulated by stretching or hormones • 6 major locations: • inside the eye 2. walls of vessels 3. respiratory tubes • 4. digestive tubes 5. urinary organs 6. reproductive organs

Muscle Histology